The methods of dulling a roll include, among others, shot-blasting, electric discharge machining, and working the roll surface by a laser.
An apparatus for dulling the roll surface by a laser is disclosed in Japanese Examined Pat. Publication (KOKOKU) Nos. 58-25557 and 60-2156.
The apparatuses disclosed in the above Pat. Publications dull the surface of a roll with a pulsed laser beam projected from a laser source of a YAG laser, ruby laser, or the like by Q-switching, and are characterized by providing a laser beam splitter which irradiates the surface of a rotating roll (Japanese examined Pat. Publication No. 58-25557) and controls which a number of laser beam pulses emitted from the laser beam splitter in accordance with a dulling shape (Japanese Examined Pat. Publication No. 60-2156).
To dull the roll surface, the roll surface must be periodically worked with a pulse laser having a predetermined repetition rate, pulse width, and peak value.
Such a pulse laser output can be provided by pulsing a continuous-wave (CW) laser output using a mechanical optical apparatus (chopper, shutter, etc.) as shown in FIG. 2, or by using a pulse laser or Q-switched laser beam (as shown in FIG. 3).
In the Q-switched pulse laser, since a mechanical device is not required, the equipment can be made compact and simple and the pulse repetition rate can be easily controlled over a wide range, in comparison with other methods. However, when the frequency is changed by the Q-switching, as shown in FIG. 4a and FIG. 4b, since the oscillation and excitation conditions vary, the pulse waveform and peak value output are also varied at the same time, and thus a stable dulling operation cannot be obtained. Also, the peak value (Pl.sub.1) of the leading pulse is very much larger than those of the subsequent pulses (Pl.sub.2, Pl.sub.3 , Pl.sub.4 , - - -), as shown in FIG. 1a.
Furthermore, the conventional Q-switched pulse laser beam generally has a fault in that an average pulse peak value (Pl.sub.p =(Pl.sub.1 +Pl.sub.2 +Pl.sub.3 +pe.sub.4)/4) is 10.sup.5 -10.sup.5 Watts (W) and a half-power width (tp) is less than 1 micro second conversion of the pulse peak value into power strength (W/cm.sup.2) is shown in FIG. 5), and the working (rater formation) domain is not suitable for dulling. Namely, working parts are evaporated, and therefore, a crater-shaped hole cannot be formed, as shown in FIGS. 7(a-1) and 7(a-2) (height h or depth d of the uneven surface is less than 1 .mu.m).
Japanese Examined Pat. Publications No. 58-25557 and No. 60-2156 mentioned above do not disclose a solution to the above problems of the conventional techniques. The dulling of a roll surface of a rolling mill is disclosed in other published documents, e.g., Japanese Examined Pat. Publication No. 61-28436 and U.S. Pat. No. 4,329,562.
The above JEPP '436 discloses a process in which two luminous fluxes are intermittently focused a lens through a circular rotating plate having a transmission zone and a reflection zone, onto the roll surface, and the above USP '562 discloses a process by which a specific motif or motif patterns are formed on the roll surface of the rolling mill.
The above publications do not disclose a solution to the above-noted problems of a Q-switched pulsed laser.